Vehicle friction brake systems, and other friction systems, such as clutches, include a plurality of friction members, some of which rotate with respect to the others. Braking is obtained by the frictional engagement between the relatively rotating members. Many of these devices utilize fluid pressure actuated pistons for accomplishing the required movement in the friction components to obtain the frictional engagement. The piston or pistons generally include an effective pressure area to which fluid pressure is applied to create a force urging the piston in a brake applying or brake releasing direction. The prior art has suggested a variety of piston retraction mechanisms to obtain brake release.
In off-highway apparatus such as construction vehicles and mining equipment, a brake system is necessary which can bear up under the severe operating conditions. The brakes on these vehicles are often subjected to extremely large braking torques and braking applications for extended periods of time. The brakes are employed not only to stop vehicle motion, but are utilized in retarding vehicle speed when traveling downhill. The brake unit must have the capability of dissipating extremely large kinetic energies developed due to the large vehicle mass, especially when the brakes are applied for extended time periods. For this and other reasons, a brake system of the multi-disc type is often chosen for this application.
A typical multi-disc brake includes a series of interleaved, non-rotatable and rotatable friction disc plates. The rotatable disc plates are operatively connected to the wheel and the non-rotatable disc plates are coupled or "grounded" to the axle housing or other non-rotating wheel support structure. Both the rotatable and non-rotatable disc plates are mounted for axial movement with respect to the axle and are enclosed within a brake housing. Spline connections are generally employed to couple the plates and the brake member to which they are operatively engaged. To obtain braking in this type of brake, the interleaved disc plates must be compressed so that their friction surfaces engage to convert the mechanical energy associated with the rotation of the rotatable disc plates into heat, which is then dissipated. The stack of interleaved disc plates is generally compressed between a wall of the brake chamber in which it is enclosed, and a movable pressure plate. In at least some brake units, a fluid pressure operated piston or pistons engage the pressure plate and move it axially into abutting contact with the outermost friction disc causing subsequent engagement of all the disc plates.
To increase the heat dissipation rate of multi-disc brakes, the discs are sometimes operated in a fluid medium which flows through the brake housing, absorbing heat from the friction discs and then transfers it to a remote heat exchanger. The multi-disc brake, coupled with a cooling system, provides a brake system having an extremely large torque capacity in a relatively small package.
Some proposed systems have suggested the use of separate retraction springs coupled to the actuating piston to obtain brake release, so that when the pressurized fluid acting on the piston was terminated, the springs would force the piston to its released position. Other systems would employ separate fluid operated retraction pistons to effect the same result. Still others have suggested the utilization of a pressure plate biased towards brake application by a plurality of springs. The piston is arranged to oppose the spring applied force when the piston is subjected to fluid pressure. This type of brake is often termed a "spring applied hydraulically released"" or "SAHR" brake.
The emergency application of the vehicle brakes upon failure of the fluid pressure system has been addressed by some prior art brake systems. Suggested mechanisms have included spring biased emergency pistons normally held in a released position by a separate fluid pressure system. Upon brake failure, the emergency pressure system would deplete the pressure applied to the spring biased piston allowing it to engage the vehicle brakes. Other systems have used redundant fluid pressure operated pistons supplied with separate sources of fluid pressure. In a SAHR type brake, a failure in the hydraulic system causing a loss in hydraulic pressure to the piston results in the application of the brake by the biasing springs. In many of these suggested systems, the apparatus added significant complexity to the brake housing and more importantly, adversely affected the brake assembly size, making them unsuitable for many vehicle applications having brake size constraints. In those systems which would suggest the use of separate source of pressurized fluid, the control system necessary to effect reliable operation would be costly to manufacture and maintain.